Remove six

[ZipFile]

We do not support Python 2 for about 5 years now, so six is no longer needed.

Bonus

Removing six actually improves performance a bit (+0.6%) and produces less boilerplate C code by Cython.

Code generated with six:

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Code generated without six:

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[coveralls]

coverage: 94.116% (-0.003%) from 94.119%
when pulling 3ba4704 on remove-six2
into aa56b70 on develop.